1. Field of the Invention
The present relates to the method for producing yttrium-aluminum-garnet material, which hereinafter is referred to as YAG or YAG material, and to a YAG base phosphor fine particles activated with terbium.
2. Description of the Prior Art
YAG is an oxide having the compositional formula of Y.sub.3 Al.sub.5 O.sub.12 and finds its application as the starting material for a laser oscillator element or as an artificial jewel. The YAG solid solution system in which yttrium is partially replaced by terbium turns out to be a phosphor or luminophor material represented by the compositional formula Tb.sub.x Y.sub.3-x Al.sub.5 O.sub.12 where x denotes a number less than three and indicates the amount of substitution. The YAG material with terbium is known to emit fluorescence efficiently when x.gtoreq.0.15. This material has a high light emission saturation level and is suited for a high luminosity phosphor screen such as that for a projection type television receiver, beam index tube or a display for an aircraft instrumentation system.
Among the factors influencing the properties of the YAG base phosphor materials are the particle size, uniformity, dispersibility and purity. Since it is thought that by optimizing these factors a cathode ray tube having a fine phosphor pattern, such as that for the recently developed high definition television system, may be produced with high resolution and high image quality, studies are being conduced of the YAG base material as a promising phosphor material.
The YAG material and the YAG base phosphor material may be synthesized by an essentially identical procedure.
One of the representative methods for synthesizing the YAG materials is a method consisting of the steps of crushing and mixing yttrium oxide and aluminum oxide in a ball mill along with a sintering assisting agent to form a mixture; subjecting the mixture to a solid-phase reaction for a predetermined time at an elevated temperature of 1500.degree. C to form a reaction product; and then crushing the reaction product to a fine particle size by a ball mill and classifying by a sieve. An improved method is essentially similar to the above method except that a flux is added during the aforementioned solid phase reaction to reduce the physical strain applied to the YAG fine particles.
For producing the YAG base phosphor material, the operation similar to that described above is performed, however, with an addition of terbium oxide to the aforementioned yttrium oxide and aluminum oxide during the crushing and mixing step.
However, in these conventional methods, the reaction product from the slid phase reaction is crushed in a ball mill to a fine particle size, the produced YAG fine particles or YAG base phosphor fine particles have a wide particle size distribution of 2 to 30 microns, so that coarse size particles are unavoidably included in the produced particulate material. On the other hand, since a prolonged time is involved in the ball-milling operation, impurities, such as metal oxides, tend to be mixed into the particulate material. There is a certain limitation in particle size reduction even with the use of the flux, such that it is usually difficult to produce particles having the particle size of less than 10 .mu.m.
On account of these problems, inconveniences, such as the deteriorated energy conversion efficiency, unnecessary coloration and/or the lowered resolution may be invited when above all the YAG base phosphor fine particles are applied to, for example, a high definition television system.